1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having solder bumps. More particularly, it relates to a method of forming a barrier metal layer between a solder bump and a pad electrode by a plating method.
2. Description of the Prior Art
In recent years, a method using solder bumps for the connection between a semiconductor chip and a substrate, or the connection between a semiconductor package and the substrate has come into wide use. Below, a description will be given to flip chip technology in which a solder bump is formed via a barrier metal on a pad electrode of a semiconductor chip as a prior art example.
In a flip chip, in order to prevent a reduction in interface strength, a monolayer or multilayer metal film (below, referred to as a xe2x80x9cbarrier metal layerxe2x80x9d) which prevent the diffusion of a solder component into the pad electrode is inserted between a solder bump and a pad electrode of a semiconductor chip. Among the components of the solder, the reactivity of tin with the barrier metal layer is high. Therefore, in the prior art, there have been used a method in which solder with low tin content is used, or a method in which the barrier metal layer is increased in thickness.
However, in recent years, lead-free solder containing Sn as a main component has become studied for preventing the environmental pollution. At present, we conduct a study of the use of a multilayer film including a Ni film, a Cr film, or an alloy layer mainly containing them as the barrier metal layer.
It is possible to improve the reliability due to the barrier metal layer by increasing the barrier metal film thickness, and increasing the diffusion length. The barrier metal layer is formed by using a sputtering method or a plating method. The plating method has a higher throughput. Accordingly, it is more suitable for forming a thick-film barrier metal layer. Therefore, the barrier metal layer is often formed by plating.
Particularly, an electroplating method allows selective growth by using a mask, and thereby offers the following advantages.
(1) It is possible to more reduce the warpage of a substrate due to film stress as compared with blanket deposition method;
(2) In blanket deposition method, in addition to a plating seed layer, a plating film must be subjected to etching processing to form a barrier metal pad. Whereas, in selective growth method, processing of only the plating seed layer is sufficient, and hence processing is easier as compared with blanket deposition method; and (3) The material cost is lower as compared with the blanket deposition method because of selective growth.
The formation process of the barrier metal layer according to the prior-art electroplating method will be described by reference to FIGS. 1A-1C. On a semiconductor substrate 10, a pad electrode 14 is provided via an insulating film 12. A passivation film 16 provided over electrodes including the pad electrode is provided with an opening on the pad electrode. On the whole surface of such a semiconductor substrate, first, as shown in FIG. 1A, an adhesion layer 20 and a seed layer 22 are successively formed by sputtering. Then, as shown in FIG. 1B, a photosensitive film 30 of photoresist commonly used in a semiconductor industry, polyimide, a dry film, or the like is directly formed on the seed layer 22. Thereafter, an opening 40 is formed therein by exposure and development. Subsequently, as shown in FIG. 1C, a plating film 50 is formed by an electroplating method in the opening 40.
The aforesaid seed layer for electroplating may be formed by using a material containing Ni or Cr, which is the same material as the plating film, as a main component, or by using a material containing Cu as a main component. The seed layer and the plating film are made of the same material for the following purpose. Namely, by using the same material, the interfacial mismatch is reduced, with the result that the interfacial adhesion and the plating film quality are improved. However, in actuality, on the seed layer surface prior to plating, an oxide film is formed. For this reason, the interface between the seed layer and the plating film becomes the seed layer oxide film/plating film. Accordingly, both the interfacial adhesion and the plating film quality are inferior.
The oxide film on the seed layer surface is formed from reaction with oxygen by exposure to atmosphere in a step after seed layer formation, and an oxygen plasma treatment before the plating step. The oxygen plasma treatment before the plating step is performed for removing the organic residue left in the opening of the photosensitive film, and for enhancing the wettability of the seed layer or the photosensitive film, and thereby improving the plating property.
When the main component of the seed layer is Ni or Cr, the oxide film is a corrosion resistant passive state film. In a conventional plating step, the surface oxide film is removed by a chemical solution of an acid or the like. However, when the oxide film is passive, it is difficult to remove. The study by the present inventor proves that the removability is improved by an increase in time, an increase in concentration, an increase in temperature, or the like of the acid treatment. It has been also shown that the interfacial adhesion and the film quality are improved at this step. However, it has been also shown that, if the acid treatment enough for sufficiently removing the oxide film is performed, the deformation of the photosensitive film and a reduction in adhesion between the photosensitive film and the seed layer cause the degradation of the photosensitive film such as peeling or immersion of the chemical solution. Namely, the amount of the oxide film removed by the acid treatment and the chemical resistance of the photosensitive film are in a relationship of trade-off. Therefore, it is difficult to completely remove the oxide film by the chemical treatment with an acid or the like. Even if the conditions for ensuring the compatibility therebetween are satisfied, the process margin is small, so that the application thereof to actual production is difficult.
In order to improve the adhesion, there is proposed the deposition by a two-stage plating process in which strike plating is performed, followed by conventional plating (Japanese Published Unexamined Patent Application No. Hei 9-186161). With this method, as shown in FIG. 2, a Ni strike plating film 50a for improving the adhesion is formed on a Ni seed layer, and thereafter, a conventional Ni plating film 50b is successively formed thereon. However, our study proves as follows. Namely, even if strike plating is performed, the adhesion is lower as compared with the case where a sufficient acid treatment has been performed. As a result, the plating layer peels off from the seed layer with ease.
On the other hand, a study has been also made on a method in which a material containing Cu as a main component is used for the seed layer. This is for the following reasons: a good plating film is expected to be formed due to the implementation of the good film quality of the seed layer; and production and development thereof are easy because Cu has a high track record as a material for buried wiring of a semiconductor. However, Cu is rich in reactivity. Therefore, for example, when polyimide is used for the photosensitive film, the interface between Cu and polyimide reacts at a temperature of 200xc2x0 C. or less to form a reaction layer. Further, an alkaline developing solution for developing the photosensitive film reacts with the Cu surface at room temperature.
It is also possible to remove the reaction layer by subjecting it to a chemical solution treatment prior to plating. However, after removal, the film thickness of the seed layer is decreased by the thickness corresponding to the reaction amount. If the film thickness is small, the sheet resistance of the seed film is increased. Therefore, the range (variation) of the electric potential distribution in the substrate plane during electroplating is increased. The deposition rate of the plating film strongly depends upon the electric potential. Accordingly, this electric potential distribution increases the range (variation) of the film thickness distribution of the plating film. In general, the film thickness of the reaction layer has a substrate in-plane distribution. This further increases the range of the film thickness distribution of the plating layer for the same reason. In a semiconductor device in which a large number of solder bumps are arranged on the whole surface of the semiconductor chip, the proportion of the area of the opening provided in the photosensitive film to the substrate area is large. Accordingly, the range of the plating film thickness distribution described above becomes noticeable.
Further, some reaction layers are difficult to remove by a chemical solution. In such a case, the reaction layer at the interface between the plating layer and the seed layer causes the degradation in the film quality of the plating layer, or the degradation in the adhesion of the interface between the plating layer and the seed layer.
As described above, the following two are the immediate problems: (1) when the seed layer forms a passive state oxide film as with Ni or Cr, high adhesion and good film quality cannot be obtained by a prior-art technique; and (2) when the seed layer reacts with the photosensitive film as with Cu, the in-plane nonuniformity of the plating film thickness is caused by a prior-art chemical removal.
In view of the foregoing circumstances, it is therefore an object of the present invention to provide a method of manufacturing a semiconductor device whereby high adhesion and good film quality can be obtained even when a seed layer forms a passive state oxide film; and good in-plane uniformity of a plating layer can be obtained even when the seed layer reacts with a photosensitive film, thereby allowing the reliability and productivity improvements.
In order to attain the foregoing object, a method for manufacturing a semiconductor device of the present invention, includes the steps of: providing a substrate having a connecting site; forming a seed layer over the connecting site; forming a protective layer on the seed layer; forming a mask on the protective layer, the mask having an opening over the connecting site; removing the protective layer to exposed the seed layer in the opening; forming a plating film on the seed layer exposed in the opening; and forming a solder bump on the plating film.
According to the present invention, the seed layer is covered with the protective layer, and the protective layer is selectively removed with respect to the seed layer immediately before the plating step. As a result, it is possible to prevent the oxidation of the seed layer in a step after formation of the protective layer. Further, it is also possible to prevent the seed layer from reacting with the photosensitive film or the developing solution of the photosensitive film. Therefore, it is possible to improve the adhesion of the plating film/seed layer interface, and it is possible to improve the substrate in-plane uniformity of the plating film thickness.